This project investigates primate biobehavioral development through comparative longitudinal investigations, with special emphasis on characterizing individual differences among rhesus monkeys in response to mild environmental challenge and on determining the long-term developmental consequences for these individuals in different physical and social environments. Studies completed in FY88 refined neonatal measures predictive of these individual differences, characterized long term influences of different early rearing environments, extended the known period of development for which continuity of these individual differences can be demonstrated, and identified parallel phenomena among wild-born rhesus monkeys living in field settings. More specifically: (1) Measures of infant state throughout the first month of life were found to be highly predictive of behavioral, neurohormonal, and immunological response to separation in both nursery reared and mother reared monkey infants and juveniles, greatly expanding the utility of such early measures for monkeys born and reared in complex social groups. (2) Differential early rearing (mother vs. nursery-peer) of rhesus monkey infants was shown to have significant behavioral, adrenocortical, neurochemical, and immunological consequences that can be detected under diverse conditions of novelty and challenge throughout the childhood and adolescence years in these subjects. (3) Continuity of individual differences in response to challenge among like-reared monkeys from infancy to adolescence and early adulthood, previously demonstrated for behavioral and adrenocortical indices, was shown to extend to measures of central monoamine turnover, with strong circumstantial evidence that such differences were highly heritable. (4) Studies of wild-born rhesus monkey groups living in naturalistic settings revealed that the basic pattern of developmental stable individual differences in biobehavioral response to challenge identified in previous laboratory studies not only generalized to natural groups but also appeared to be of considerable biological significance for these monkeys.